1. Field of the Invention
The present invention relates to a heat generating element having at least one PTC heating element and an insulated housing surrounding the PTC heating element as well as electric strip conductors lying against the PTC heating element at opposite sides.
2. Description of the Related Art
Such a heat generating element is known as part of an auxiliary heater for a motor vehicle, for example from EP 0 350 528. Other heat generating elements are known, for example, from DE 32 08 802, DE 30 46 995 or DE 28 04 749.
Basically, in such generic heat generating elements, there is a problem in that a low transition resistance is to be provided by good mechanical contacting between the strip conductor and the PTC element, such that current feed to the heat generating element is possible without substantial heating at the phase interface to the PTC element. This requirement becomes in particular relevant if the heat generating element is to be fed with high operating voltages of approx. 500 Volts or more.
In generic electric heating devices, the strip conductor, which is usually formed by an electrically conductive sheet metal strip, is encapsulated by a sleeve surrounding the heat generating element, which sleeve places the strip conductor against the at least one PTC element with some pressure (as in DE 32 08 802). In this prior art, the PTC element with the strip conductors abutting on both sides is surrounded by a metallic sleeve which is coated with silicone rubber inside, so that the conducting sheet metal strips are held in the sleeve in an insulating manner. This arrangement alone is not sufficient for developing sufficient contact pressure for pressing the strip conductors against the PTC element. Correspondingly, the complete layer composition is surrounded by a press plate. The known heat generating element is relatively passive, i.e. the conduction of heat generated by the PTC element to the outside is relatively poor. The known heat generating element correspondingly has a poor thermal efficiency and reacts relatively slowly to changing thermal conditions.
For heat dissipation, it is known for example from EP 0 350 528 to place radiator elements formed by sheet metal strips bent in a meandering manner against both sides of the heat generating element. These radiator elements are placed against the heat generating element under spring pretension. As the strip conductor between the radiator element and the at least one PTC element is provided so as to be freely movable, the strip conductor is placed against the PTC element by means of the spring force. This construction, however, involves the problem that in particular in operation of the heat generating element at high voltages, leakage currents migrating via the radiator elements and/or the frame cannot be prevented. Moreover, the current-bearing parts at the outer side of the heat generating element are exposed, which is critical also for safety reasons.
The heating rod known from DE 28 04 749 also has the above mentioned disadvantage with respect to poor heat conduction, in this heating rod three generic heat generating elements being arranged at an angle of 120° offset to each other about a cylinder axis. Between the individual heat generating elements, there are located cylindrical segment parts of an electric insulating material in which one flow channel each is recessed for a fluid to be heated by the heating rod. Such a construction is insufficient in particular in the convective dissipation of heat generated by the PTC element by air. Here, heat cannot be dissipated from the PTC element to the extent required.
The problem underlying the present invention is to provide a heat generating element wherein good contacting between the strip conductor and the at least one PTC element can be ensured and which is electrically insulated with respect to the surrounding area in an improved manner. Moreover, with the present invention, an improved electric auxiliary heater for a motor vehicle is to be provided.